The present invention relates to a process for conversion of a feed hydrocarbon stream such as natural gas to liquid hydrocarbon products. More specifically, the invention relates to a process comprising the use of a cryogenic air separation unit (ASU), optionally together with vacuum pressure swing adsorption (VPSA), to provide oxygen with purity of at least 80% vol. The feed hydrocarbon stream is combined with steam, optionally with subsequent pre-reforming, and passed with the oxygen through an autothermal reformer (ATR) or catalytic partial oxidation (CPO) to produce a raw synthesis gas, of which a portion is passed through a pressure swing adsorption (PSA). The raw synthesis gas is used in downstream Fischer-Tropsch synthesis for production of liquid hydrocarbons, including production of a Fischer-Tropsch tail gas stream which is recycled to e.g. the feed hydrocarbon stream prior to or after addition of steam, or to the ATR or CPO. The invention further relates to a plant for carrying out the process. The process and plant are particularly suitable for small GTL plant(s) producing e.g. 500-5000 BPD of liquid hydrocarbons, in particular 1000-3000 BPD.
It is well-known to use autothermal reforming (ATR) for producing synthesis gas from e.g. natural gas with downstream production of liquid hydrocarbons, since ATR technology is capable of producing synthesis gas with the right H2/CO molar ratio of about 2 required for downstream Fischer-Tropsch synthesis used for producing liquid hydrocarbons, in particular diesel.
While the combination of ATR and Fischer-Tropsch is normally used in large plants, also called gas to liquid (GTL) plants where large natural gas reserves exist, there is an increasing demand to produce liquid hydrocarbons from much smaller natural gas reserves, i.e. by providing small GTL plants. This development is at least partly driven by poor pipeline accessibility to the remote places where such smaller natural gas reserves (fields) are located. Hence, it is desirable to be able to convert natural gas to liquid hydrocarbons, as the latter is easier to transport. Apart from natural gas, associated gas, also called associated petroleum gas (APG), may also suitably be converted to liquid hydrocarbons, instead of being flared.
For small GTL plants producing about 1000 BPD (barrels per day) from a natural gas feed, it is known from U.S. Pat. No. 9,034,208 to combine the use of vacuum pressure swing adsorption, autothermal reforming, and hydrogen removal by means of a hydrogen-membrane while using the tail gas from Fischer-Tropsch synthesis (Fischer-Tropsch tail gas) as fuel to preheat the feeds supplied to the autothermal reforming stage.